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Re: [Phys-L] inertia and the tablecloth demo



I see two issues that make it easy for students to miss this question.

First, as beginners with the third law, unless you teach them explicitly to
think of weight as "the Earth pulls on the object", they are going to have
trouble fitting weight into their framework for this law. (And using the
term "true weight" at this point in the course just raises more questions
that we have not yet addressed.)

Second, the phrase "equal and opposite" shows up in two distinct contexts
that are easy to confuse. In this case, there is in fact an equal and
opposite force to the pull of gravity -- the upward force by the table. It
exists, it is equal and opposite, but it is not the one that you asked
for. I think many middle schools teach students to recite a version of the
third law. So they come in already knowing the words: "For every force
there is an equal and opposite force". You have to explicitly point out
that this law cannot be talking about balancing forces! If every force had
a balancing force, nothing could accelerate. So this "equal and opposite"
is talking about something else.

I do as questions like this on multiple choice tests. But I warn students
in advance and then again on the test itself, in the text of the item: I am
not asking you to identify an equilibrant but rather a "reaction" force as
required by the third law.

On Sat, Aug 20, 2016 at 5:35 AM, John Denker <jsd@av8n.com> wrote:

On 08/19/2016 08:46 PM, Anthony Lapinski wrote:
I first teach about true weight (mg -- downward) and apparent weight
(scale
reading, normal force -- upward).

I'm sure others teach weight differently as this discussion has
come up before on this listserv.

I for one would have a hard time with the idea of an "upward" direction
assigned to the apparent weight, or to any other kind of weight.

Where I come from, the weight vector /defines/ what we mean by "down".

The MC answers are constructed so there
is no ambiguity (earth on book, book on earth, table on book, book on
table).

That looks ambiguous to me.

I predict that students who do well on other parts of the test but
get this question wrong choose "table on book" significantly more
than 33% of the time.

Rationale: weight = book on table, counterforce = table on book.

Another rationale: The table-on-book force is /equal/ to the
book-on-earth force. Mathematically they are the same vector,
since they have the same magnitude and same direction. To consider
one of them right and the other wrong is literally a distinction
without a difference.

============

I see this sort of thing all the time. Usually it is intentional:
I assign a question that is known to have eight different correct
answers.

Occasionally it is unintentional: Suppose I really want and expect
a particular answer and a particular method of solution, to make an
important point or exercise an important technique. If some student
finds an unexpected solution, I cheerfully mark it correct. (I reserve
the right to ask followup questions so that the important points get
covered eventually.)

What I /expect/ has no bearing on the correctness of the student's
work. It's not an ESP exam.

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